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Lopsided Collections of Satellite Galaxies

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Andromeda
The Andromeda galaxy has many satellite galaxies, including the two brightest visible in this photo, M32 and M110 (click for the full view!). A new study examines how satellite galaxies are distributed around their hosts throughout the universe. [Boris Štromar]

You might think that small satellite galaxies would be distributed evenly around their larger galactic hosts — but local evidence suggests otherwise. Are satellite distributions lopsided throughout the universe?

Satellites in the Local Group

The distribution of the satellite galaxies orbiting Andromeda, our neighboring galaxy, is puzzling: 21 out of 27 (~80%) of its satellites are on the side of Andromeda closest to us. In a similar fashion, 4 of the 11 brightest Milky Way satellites are stacked on the side closest to Andromeda.

It seems to be the case, then, that satellites around our pair of galaxies preferentially occupy the space between the two galaxies. But is this behavior specific to the Local Group? Or is it commonplace throughout the universe? In a recent study, a team of scientists led by Noam Libeskind (Leibniz Institute for Astrophysics Potsdam, Germany) set out to answer this question.

Properties of the galaxies included in the authors’ sample. Left: redshifts for galaxy pairs. Right: Number of satellite galaxies around hosts. [Adapted from Libeskind et al. 2016]

Properties of the galaxies included in the authors’ sample. Left: redshifts for galaxy pairs. Right: Number of satellite galaxies around hosts. [Adapted from Libeskind et al. 2016]

Asymmetry at Large

Libeskind and collaborators tested whether this behavior is common by searching through Sloan Digital Sky Survey observations for galaxy pairs that are similar to the Milky Way/Andromeda pair. The resulting sample consists of 12,210 pairs of galaxies, which have 46,043 potential satellites among them. The team then performed statistical tests on these observations to quantify the anisotropic distribution of the satellites around the host galaxies.

Libeskind and collaborators find that roughly 8% more galaxies are seen within a ~15° angle facing the other galaxy of a pair than would be expected in a uniform distribution. The odds that this asymmetric behavior is randomly produced, they show, are lower than 1 in 10 million — indicating that the lopsidedness of satellites around galaxies in pairs is a real effect and occurs beyond just the Local Group.

Caution for Modeling

Probability that satellites are located at an angle of θ degrees from the direction pointing toward the other galaxy in the pair. There are more satellites found in the space between the pair than predicted by a uniform distribution. [Libeskind et al. 2016]

Probability that satellites are located at an angle of θ degrees from the direction pointing toward the other galaxy in the pair. There are more satellites found in the space between the pair than predicted by a uniform distribution. [Libeskind et al. 2016]

What might cause this asymmetric distribution? The authors suggest the primary cause is that galaxies in pairs are not necessarily relaxed halos in equilibrium — a case in which spherical symmetry would apply. Instead, these are likely merging, dynamically active pairs of galaxies, so we cannot assume that they have axially symmetric halos.

Simulations of Local-Group-like pairs of galaxies will be the next step needed to understand how such asymmetries in the distribution of satellites form and evolve. Meanwhile, the results presented here suggest that the commonly adopted axially symmetric models of the Milky Way (and other galaxies in pairs) should be used with caution, as they may not be capturing the true shape of the halo.

Citation

Noam I. Libeskind et al 2016 ApJ 830 121. doi:10.3847/0004-637X/830/2/121